1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus capable of forming a quality color image by effectively correcting a variation in surface moving speed of a surface moving member.
2. Discussion of the Background
Image forming apparatuses include copiers, printers, facsimiles, multi-function devices thereof, etc. Some image forming apparatuses form a color image on a recording member according to an electrophotographic method. Such image forming apparatuses optically write a latent image on an image carrier, develop the latent image with toner, transfer the toner image onto a recording member carried on a surface moving member to obtain a color image.
Alternatively, such image forming apparatuses optically write a latent image on a surface moving member, develop the latent image with toner, temporarily transfer the toner image onto a surface moving member and then transfer the toner image onto a recording member carried on another surface moving member to obtain a color image.
In recent years, high image quality and high speed processing have been increasingly demanded for the above image forming apparatuses. The image forming apparatuses capable of meeting such demands include a tandem-type image forming apparatus employing a direct transfer method.
For example, a tandem-type image forming apparatus employing a direct transfer method forms single-color toner images of black, yellow, magenta, and cyan on photoconductive drums for black, yellow, magenta, and cyan, respectively. Then, the tandem-type image forming apparatus superimposingly transfers the single-color toner images onto a recording member carried on a conveyor belt.
However, this tandem-type image forming apparatus employing the direct transfer method may cause a noticeable color misregistration. The color misregistration is caused by a relative displacement in transfer position between the single-color toner images formed on the recording member. Further, the color misregistration may cause bleeding between line images to be precisely superimposed on each other, or a white spot around a black-color character on a multi-color background. The color misregistration may also cause banding, that is, periodic band-shaped unevenness in toner density on a color background.
In addition, the image forming apparatuses capable of meeting such demands include a tandem-type image forming apparatus employing an intermediate transfer method. For example, a tandem-type image forming apparatus employing an intermediate transfer method forms single-color toner images of black, yellow, magenta, and cyan on photoconductive drums for black, yellow, magenta, and cyan, respectively. Then, the tandem-type image forming apparatus superimposingly transfers the single-color toner images on an intermediate transfer belt. Then, the tandem-type image forming apparatus transfers a resultant color image onto a recording member.
However, this tandem-type image forming apparatus employing the intermediate transfer method may also cause a noticeable color misregistration similar to the tandem-type image forming apparatus employing the direct transfer method. The color misregistration may be caused by a relative displacement in transfer position between the single-color toner images formed on the intermediate transfer belt.
The color misregistration is also caused mainly by a relative displacement in transfer position between single-color toner images formed on each of the photoconductive drums. Further, the relative displacement in transfer position between single-color toner images formed on each of the photoconductive drums is caused by a periodic variation in surface moving speed of each of the photoconductive drums.
Furthermore, the periodic variation in surface moving speed of each of the photoconductive drums may be significantly influenced by a variation in rotation angular speed of a rotation drive force transmitted to each of the photoconductive drums. The variation in rotation angular speed may be caused by a transmission error due to gear eccentricity or accumulated gear pitch error of a drive transmission system provided at a shaft of the photoconductive drum. The variation in rotation angular speed may also be caused by a transmission error due to shaft inclination or misalignment of a coupling member to allow the photoconductive drum to be attached to and detached from the drive transmission system.
Some of the image forming apparatuses are capable of correcting the color misregistration by suppressing the periodic variation in surface moving speed of the photoconductive drum. For example, an image forming apparatus detects a periodic variation in surface moving speed of each of a plurality of photoconductive drums. Based on a detection result of the periodic variation, the image forming apparatus finely adjusts a rotation angular speed of each of the plurality of photoconductive drums so as to suppress the periodic variation.
Specifically, a single-color adjustment pattern having a plurality of toner images is formed on each of the photoconductive drums for black, yellow, cyan, and magenta. The single-color adjustment pattern formed on each of the photoconductive drums is transferred onto an intermediate transfer belt so as to be arranged in an order of black, yellow, cyan, and magenta toner images. Then, the resultant multi-color adjustment pattern is sequentially sensed with a pattern sensor in the order of black, yellow, cyan, and magenta toner images. From signals sensed with the pattern sensor, a periodic variation component in surface moving speed having a rotation period of the photoconductive drum are detected as detection data.
Based on the detection data, a rotation angular speed of each of the photoconductive drums is finely adjusted so as to suppress the periodic variation in surface moving speed thereof. At this time, the image forming apparatus performs the fine adjustment of rotation angular speed of each of the photoconductive drums in a manner as follows.
The above detection data is based on a detection result of the adjustment pattern formed on the intermediate transfer belt while being influenced by the following two variations. The first variation is a variation in surface moving speed of the photoconductive drum at a time when a latent image is written onto the photoconductive drum to form the adjustment pattern. The second variation is a variation in surface moving speed of the photoconductive drum at a time when the adjustment pattern obtained by developing the latent image is transferred onto the intermediate transfer belt.
In addition, this image forming apparatus is configured to have a phase difference angle of substantially 180° between a latent-image writing position and a transfer position on the photoconductive drum. Specifically, the phase difference angle is formed with two imaginary lines connecting a rotation center of the photoconductive drum to each of the latent-image writing position and the transfer position on the surface thereof on an imaginary plane perpendicular to a rotation axis of the photoconductive drum.
Accordingly, a correction value is obtained by adding up a half gain to the detection data and inverting a phase thereof. Further, the correction value is superposed on an uncorrected target value of the rotation angular speed of the photoconductive drum. Then, drive of the photoconductive drum is controlled according to the corrected target value so as to suppress the periodic variation in surface moving speed of the photoconductive drum having a rotation period thereof.
However, as described above, the image forming apparatus is configured to have a phase difference angle of substantially 180° between the latent-image writing position and the transfer position on the photoconductive drum. If the image forming apparatus is configured to have a phase difference angle significantly deviated from 180°, an inaccurate correction value may be obtained, thereby causing an error in drive control of the photoconductive drum. Consequently, some limitation may be imposed on the layout of units in the image forming apparatus.
For the above reasons, demand is increasing for an image forming apparatus capable of effectively correcting the periodic variation in surface moving speed of the photoconductive drum with less limitation to the phase difference angle.